Unmanned underwater vehicles are known for a long time. In particular, a toy submarine boat has been disclosed in the document U.S. Pat. No. 741,581. More recently, such vehicles have been used as drones for carrying out a variety of tasks in the underwater environment, e.g. pipeline surveys and investigations or military tasks. An example of such an application has been disclosed in the document WO 2014/113121.
Unmanned underwater vehicles often have small or miniature dimensions so as to permit their easy handling and their storage inside limited space. However, such miniature dimensions can increase the vulnerability of the underwater vehicles to debris intrusion or to fishing lines becoming tangled in the propeller of said vehicles, which could significantly reduce their performance and, in the worst case, lead to their deterioration and loss of the vehicle. Additionally, it can be challenging to ensure that the moving parts of a propulsion system are water- and pressure proof with high reliability. Dynamic seals around motor shafts can wear out quickly, particularly when exposed to suspended sand particles or other particulates in the water. Exposed metal parts are also affected by corrosion when exposed to water, especially in salt water. A failed seal generally results in rapid water ingress which will damage internal components, and decrease vehicle buoyancy, which may lead to a complete loss of the entire vehicle.
Different solutions to these problems have been proposed in the past.
One solution to render the electric motor of the propelling device of said vehicles water- and pressure-proof is to use O-rings and dynamic seals which may wear quickly. Filling the motor with oil to resist external pressure decreases stress on the seal and mitigates the risk of water ingress, but it increases friction within the narrow magnetic gap of the motor, may release oil into the environment and requires regular maintenance to replace lost oil. Alternatively, a magnetic coupler between the motor and the propeller can be used which adds bulk and complexity and limits torque.
A first aim of the present invention is therefore to provide an underwater propelling device for miniature underwater vehicles, wherein the above mentioned drawbacks are avoided.
A second aim of the present invention is to provide an underwater propelling device having a compact and low profile structure so as to reduce drag caused by the bulk of the motor, thus improving the underwater vehicle's performance, particularly when the propelling device is used as an attitude control thruster and mounted perpendicularly to the main direction of travel of the underwater vehicle.
A third aim of the present invention is to provide an underwater propelling device that can be easily manufactured.